1. Technical Field
The present invention relates to a distributed power supply system, such as a photovoltaic system or wind power system, that supplies power in conjunction with a power system.
2. Description of Related Art
To date, a distributed power supply system, such as a photovoltaic system or wind power system connected to a power system, which converts direct current power generated by sunlight, wind power, or the like to alternating current power, has been developed.
FIG. 6 is a block diagram for illustrating this kind of distributed power supply system. Although each of the system bus bars and signal lines is normally depicted with two or three (or four) lines, representing a two or three-phase circuit, they are depicted here with one representative line for the sake of simplification.
In FIG. 6, reference numeral 1 is an alternating current power supply that supplies power to a power system, 2 to 4 are switches, 10 is an inverter circuit for grid connection, 11 is a direct current power supply such as a photovoltaic cell, 100 is a control circuit that controls the inverter circuit 10, 5 is a load to which power is supplied from the power system of the alternating current power supply 1, 6 is a voltage detector that detects the voltage of the power system, 7 is a current detector that detects the output current of the inverter circuit 10, 8 is a capacitor, and 9 is a reactor.
The inverter circuit 10 is controlled by the control circuit 100, carries out a pulse width modulation (PWM) of the direct current voltage output by the direct current power supply 11, and emits three-phase alternating current voltages at an output terminal thereof. After its harmonic component is removed by an LC filter configured of the reactor 9 and capacitor 8, the three-phase alternating current voltage emitted by the inverter circuit 10 is transmitted to the power system to which power is supplied by the alternating current power supply 1. The control circuit 100 includes a phase-locked loop (PLL) computation unit 12, a three-phase voltage command generation unit 13, a coordinate conversion unit 14, an output current control unit 15, and a gate signal generation circuit 16. The PLL computation unit 12 has a function of generating an angular frequency ωo coinciding with a phase of the system voltage detected by the voltage detector 6. A description of a specific action of the PLL computation unit 12 will be given hereafter.
The three-phase voltage command generation unit 13 generates three-phase voltage commands Vuref, Vvref, and Vwref with a predetermined amplitude based on the angular frequency ωo output from the PLL computation unit 12. Meanwhile, the coordinate conversion unit 14 converts the coordinates of an active current command Idref and reactive current command Iqref using the angular frequency ωo output from the PLL computation unit 12, thereby generating a U-phase output current command Iuref and W-phase output current command Iwref.
The output current control unit 15 carries out an alternating current ACR control in such a way that the U-phase and W-phase output current commands Iuref and Iwref output from the coordinate conversion unit 14 equal output currents Iu and Iw of the inverter circuit 10 detected by the current detector 7, respectively. As a result of the alternating current ACR control, the output current control unit 15 generates correction amounts ΔVuref, ΔVvref, and ΔVwref for correcting the voltage commands Vuref, Vvref, and Vwref of respective phases output by the three-phase voltage command generation unit 13.
The gate signal generation circuit 16 adds the correction amounts ΔVuref, ΔVvref, and ΔVwref output from the output current control unit 15 to the voltage commands Vuref, Vvref, and Vwref of respective phases output from the three-phase voltage command generation unit 13, thereby generating a modulation signal for each phase. Next, the gate signal generation circuit 16 carries out a pulse width modulation (PWM) computation using the generated modulation signal for each phase and a carrier signal. The result of the PWM computation is used as the gate signals that control the inverter circuit 10. This kind of control method is disclosed in, for example, Electrical Engineers Symposium on Static Apparatus (Oct. 23, 2001), thesis number SA-01-39.
However, this kind of distributed power supply system is required to stably supply power to the power system. For this reason, the control circuit 100 carries out control of the frequency and phase of the voltage output from the inverter circuit 10 based on the phase and frequency of the power system voltage. In order to realize this kind of control, the control circuit 100 of the distributed power supply system shown in FIG. 6 includes the PLL computation unit 12.
A block diagram of a PLL computation unit disclosed in JP-A-2010-161901 is shown in FIG. 7 as an example of the PLL computation unit 12 shown in FIG. 6.
The PLL computation unit 12 includes an αβ conversion unit 121, a dq conversion unit 122, a proportional integral regulation unit 123, and a voltage controlled oscillator (VCO) unit 124. In the following description, the proportional integral regulation unit is also called the PI regulation unit.
The αβ conversion unit 121 converts three-phase voltage signals Vu, Vv, and Vw input from the voltage detector 6 into two-phase voltage signals Vα and Vβ. The voltage signals Vα and Vβ are input from the αβ conversion unit 121 into the dq conversion unit 122, and a phase signal θ is input from the VCO unit 124 into the dq conversion unit 122. The dq conversion unit 122 calculates a phase difference component Vd and an in-phase component Vq from the phase signal θ and voltage signals Vα and Vβ. The PI regulation unit 123 carries out a computation control with a proportional integral regulator (PI regulator) in such a way that the phase difference component Vd becomes zero, and outputs a correction value. A corrected angular frequency ωo obtained by adding the correction value to a system voltage signal target angular frequency ωs* with an adder 126 is output to the VCO unit 124. The VCO unit 124 outputs a phase θ according to the input corrective angular frequency ωo to the dq conversion unit 122.
With this feedback control, the phase difference component Vd is locked as it reaches zero. At this time, the phase θ coincides with the system voltage phase. Consequently, the corrected angular frequency ωo output from the PLL computation unit 12 coincides with the system voltage angular frequency.
When an error occurs in the system, however, the grid connection regulations (JEAC9701-2006) issued by the grid connection special committee of The Japan Electric Association require that the distributed power supply system should be stopped once before being restarted. For this reason, the distributed power supply system includes a protective function that detects a system error. Consequently, when an error such as a momentary drop in voltage occurs in the system, there is a possibility of a large number of distributed power supply systems connected to the same system being simultaneously disconnected from the system. In this case, there is a concern that a drop in the system frequency or a fluctuation in the system voltage will be caused. For this reason, it is desirable that the distributed power supply system continues to operate stably even when a momentary drop in voltage occurs for a time period that is shorter than that recognized as a system error mandated by the grid connection regulations.
However, the PLL computation unit 12 included in the distributed power supply system shown in FIG. 6 includes the PI regulation unit 123. At the PI regulation unit 123, an amount proportional to the deviation from a predetermined value is added to an amount obtained by integrating an amount obtained by multiplying the deviation by a predetermined amount, and the resulting value is used as the correction value to make the deviation of the input signal zero. That is, the PI regulation unit 123 has an integrating function, and because of this, it cannot instantly cause the output to keep up with a sudden change of the input voltage signal. For this reason, it is known that when the system voltage momentarily fluctuates due to a short circuit occurring between the phases of the power system or the like, a large phase difference occurs between the system voltage and the voltage output from the inverter circuit 10 for a short period of time. As a result of this, an excessive current caused by the phase difference occurs between the power system and inverter circuit 10, and the distributed power supply system stops.